1. Field of the Invention
The present invention relates to heterojunction field effect transistors (HFETs) and more particularly to methods for fabricating ultra-short T-gates on HFETs.
2. Brief Description of Prior Developments
Microwave and millimeter wave transistors often utilize shorter gate lengths to improve their performance and increase the frequency of operation. Ultra-short T-gate technology enhances HFET gain and low noise performance for a wide range of military and commercial systems and applications. Typically, T-gate technology employs multiple resist layers with different electron beam sensitivities for subsequent layer developing. This technique has been successfully used in our 0.1 μm HFET process, but becomes difficult to implement for sub-0.1 μm T-gates with good process control or satisfactory yield. Sub-0.1 μm T-gates are very difficult to reproduce in a multiple resist layer scheme because the electron beam's spot size and dose for the lower “stem” or “foot” are broaden by the top resist layers. An improved way to address these fabrication issues is, however, still needed.